Electric motors commonly include a stationary component called a stator and a rotating component called a rotor. The rotor rotates within (or around) the stator when the motor is energized with a driving waveform. Induction motors, sometimes referred to as asynchronous motors, are a type of electric motor wherein power is supplied to the rotor by means of electromagnetic induction rather than by means of direct electrical connections to the rotor.
As with synchronous motors, the driving waveform supplied to an induction motor's stator creates a magnetic field that rotates in time with the AC oscillations of the driving waveform. The induction motor's rotor rotates at a slower speed than the stator field. This difference in rotational speed, also referred to as “slip,” “slip frequency,” or “slip speed,” results in a changing magnetic flux in the rotor windings that induces currents in the rotor windings. The induced currents in turn generate magnetic fields in the rotor windings that oppose the rotating magnetic field created by the stator, thereby inducing rotational movement in the rotor. The rotor accelerates until the magnitude of induced rotor current and rotor torque balances the applied load. Since rotation at synchronous speed would result in no induced rotor current, an induction motor always operates at less than synchronous speed during normal forward operation.
Rotation of an induction motor may be stopped by simply removing the driving waveform from the motor and allowing the motor to coast to a standstill over time due to the inertia of the rotor and anything coupled to the rotor. Alternatively, rotation of an induction motor may be stopped more quickly using a braking method involving adjusting the frequency of the driving waveform to be less than the rotor frequency, wherein the rotating magnetic field created by the stator induces rotational pressure on the rotor that opposes the rotor's movement and actively reduces motor speed. Using this braking method, the inertia of the rotor and applied load induces voltage in the stator that may energize external motor components, such as a DC bus supplying power to the motor.
In many motor applications, it is desirable to stop rotation of the rotor as soon as the driving waveform is removed from the motor. For example, in washing machine applications, it is desirable to stop rotation of the washing machine motor after a high speed spin cycle so that the washing machine may be unloaded or switched to a slower speed wash or rinse cycle. Unfortunately, the braking method mentioned above suffers from limitations that may render it ineffective to quickly stop heavy loads. The braking pressure created by the driving waveform, for example, is limited by the electric power available to energize the stator. The braking pressure is further limited by the capacity of the motor and other components to handle the voltage induced by the rotor during the braking process.
Accordingly, various alternative techniques have been developed for braking electric motors. One such technique uses brake pads, pulleys, and/or other friction braking systems. Unfortunately, friction brakes add cost and complexity to a motor and are therefore not desirable for low cost applications such as washing machines. Friction brakes also eventually wear out with use and require repair or replacement.
Thus, many motor applications employ alternative electric braking systems rather than friction brakes. One type of electric braking system involves DC injection braking in which a direct current (DC) voltage is applied to a motor's stator windings to brake the rotor. The DC voltage creates a stationary magnetic field which applies a static torque to the rotor. This slows and eventually halts rotation of the rotor. As long as the DC voltage remains on the stator windings, the rotor is held in position and resists rotation. DC injection braking is relatively simple, cost-effective, and maintenance free and is therefore a popular choice of braking for many motor applications; however, it has not been used effectively in some applications as described below.
It is also often desirable to determine when a motor's rotor has stopped rotating so the rotor can be driven in the opposite direction, at a different speed, etcetera. This can be accomplished with a motor shaft sensor such as a Hall effect sensor, but such sensors increase the cost and complexity of motors and are therefore not desirable for many lower cost applications such as washing machine motors.
Thus, sensorless techniques for determining motor speed have been developed. One type of sensorless speed detection employs various algorithms for estimating when a rotor stops based on measured electrical parameters. However, the measured electrical parameters, and thus the results of the algorithms, are less accurate when the motor is being braked with the above-described DC injection braking techniques. Thus, DC injection braking techniques generally require a motor shaft sensor.
The above section provides background information related to the present disclosure which is not necessarily prior art.